Large screen televisions have not made deep in-roads into the consumer market for a number of reasons. Currently, the largest cathode ray tubes are approximately thirty-five inches measured diagonally and are very heavy, i.e. over 100 pounds. Vacuum technology limits the size of the screen. The larger the screen, the more glass is needed to keep the vacuum intact and the heavier the screen gets. Direct view tube technology, after serving as the work-horse of the industry, has reached its limits when it comes to larger screens. The big screen televisions currently being marketed are of the projection type. These screens use a small, extremely bright source (either a cathode ray tube or a transmissive liquid crystal display screen) and magnify it to a large size using conventional optics. Besides being bulky, such screens have a number of practical limits. There exist tradeoffs between source brightness, tube lifetime, optical aberrations and viewing angle that generally result in poor image quality for such systems, as compared to direct view cathode ray tube televisions.
In essence, what one would like is a screen technology that scales up better with size. Twenty five inch tubes cost orders of magnitude less to produce than a forty inch tube which has three times the screen area of a conventional size tube. Tube technology does not scale well, and thus what one needs is a screen that costs a fixed amount per square foot. In addition, although most televisions today are sold as large boxes, one would expect large TVs of the future to be flat screens, simply because a six foot screen contained in a six foot square box is not very practical, but a six foot screen, mounted on a wall and taking up only wall space, would be practical.
A number of flat panel technologies intended for large screen home use have been explored in the last twenty years or so, but each has its own problems. Liquid crystal displays are most often seen in calculators and lap-top computers. The liquid crystal is sandwiched between two polarizers and introduces a voltage controlled polorization rotation of the incident light. Thus, elements of the liquid crystal display can be made clear or opaque, simply by applying a voltage. The problem with this technology is that it does not scale very well. The amount of phase rotation, in addition to being proportional to the applied voltage, is also proportional to the distance between the front and back polarizers. This distance must be controlled to a few tens of microns across the entire area of the screen. Unfortunately this is virtually impossible because large screens are simply not sufficiently rigid, i.e. they flex under gravity. Making such screens thicker is not a viable solution because thicker screens get even heavier.
The LCD is a voltage controlled device. In order to selectively turn "on" one pixel, a voltage, V.sub.x, must be applied to a horizontal electrode (corresponding to that pixel) and another voltage, -V.sub.y, must be applied to the vertical electrode, so that the pixel sees a voltage drop of V.sub.x +V.sub.y. Note that if all other horizontal and vertical electrodes are held at ground potential, the screen will have other pixels with V.sub.x and V.sub.y across them. Thus, with a voltage controlled device, "crosstalk" occurs, i.e., turning "on" one pixel slightly turns "on" other neighboring pixels.
The LCD industry has gotten around this by making an "active matrix" LCD where each pixel has its own transistor driver. This resolves the crosstalk problem but introduces severe manufacturing constraints. Chip lithography (accurate to .about.1 um alignment) across a large screen (.about.1 m.sup.2) with multiple mask layers is nearly impossible and not very cost effective since the yield is very low. Ideally, what one wants is a directional current device (a diode) at each pixel to eliminate crosstalk, no critical alignments, and robustness against pixel fabrication errors. This fact has limited liquid crystal display screens to small sizes, such as less than thirteen inches diagonally.
Another flat panel technology that suffers from this same problem is plasma display technology. In this technology, exciting electrodes must be properly spaced across a large area. This is not a trivial task. In addition, plasma displays require high driving voltages, i.e. a few kilivolts. The nature of flat panel technology requires X-Y addressing electrodes, one for each horizontal and vertical line of resolution. Thus a thousand by thousand pixel screen requires two thousand drive lines. Plasma displays need high voltage drivers (i.e., high voltage semiconductors) for every line, making them prohibitively expensive for home use.
Another major flat panel technology consists of electroluminescent screens made of phosphors, sandwiched between X-Y electrodes. This device is solid and does not have plate separation problems like the previously discussed approaches. It does, however, require high voltage drivers to excite the phosphors, thereby again making high resolution infeasible for home use.
There is therefore an ongoing need to provide a way of overcoming the obstacles of the noted previous technologies to provide an improved display screen and method of manufacture thereof, which is especially conducive to the manufacture of large screen televisions without suffering the aforementioned disadvantages of the prior art.